Vocal communication among members of free ranging populations of rats housed in complex laboratory enclosures are either audible to humans, or are above 20 kHz, the upper limit of human hearing (ultrasonic). Audible vocalizations are frequently uttered as one, or a short series of squeals by a rat when attacked or confronted by another. Ultrasonic vocalizations are uttered during attacks and confrontations during naso-naso, naso-anal, and naso-genital inspections, and as part of male post-copulatory behavior. When two, or more rats are vocalizing simultaneously they are capable of utilizing separate frequency bands so that each signal is distinguishable. Vocalizations of a population forced to perform cooperative behaviors to obtain food and water are being compared to those of a population for which food and water are freely available. An extensive library of vocalizations is being prepared for correlation with direct observations of behavior and computer records of movement patterns to determine if those rats required to perform cooperatively exhibit different and, possibly, more complex patterns and vocalization and behavior than those rats with free access to food and water.